In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. In particular, to provide a competitive alternative to conventional cinematic-quality projectors, digital projection systems need to provide high resolution, wide color gamut, high brightness (>10,000 screen lumens), and frame-sequential system contrast ratios exceeding 1,000:1. In addition, the digital systems must also provide constancy of image quality, image data security, low equipment purchase and maintenance costs, and low data distribution costs, to make a switchover from conventional film based systems compelling.
As discussed in commonly-assigned U.S. Pat. No. 6,585,378 (Kurtz et al.), liquid crystal displays (LCD) can be used in the construction of digital cinema projection systems. The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. Among other examples of electronic projection apparatus that employ LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.) and U.S. Pat. No. 5,918,961 (Ueda). A few years ago, JVC demonstrated an LCD-based projector capable of high-resolution (providing 2,000×1,280 pixels), high frame sequential contrast (in excess of 1,000:1), and high light throughput (nominally, up to 12,000 lumens). This system utilized three vertically aligned (VA) (also referred as homeotropic) LCDs (one per color) driven or addressed by cathode ray tubes (CRTs).
JVC has also developed a new family of vertically aligned LCDs, which are directly addressed via a silicon backplane (LCOS), rather than indirectly by a CRT. The JVC LCD devices are described, in part, in U.S. Pat. No. 5,652,667 (Kuragane) and U.S. Pat. No. 5,978,056 (Shintani et al.) In contrast to most twisted nematic or cholesteric LCDs, vertically aligned LCDs promise to provide much higher modulation contrast ratios (in excess of 2,000:1). It is instructive to note that, in order to obtain on screen frame sequential contrast of 1,000:1 or better, the entire system must produce >1,000:1 contrast, and both the LCDs and any necessary polarization optics must each separately provide ˜2,000:1 contrast. Notably, while polarization compensated vertically aligned LCDs can provide contrast >20,000:1 when modulating collimated laser beams, these same modulators may exhibit contrasts of 500:1 or less when modulating the same collimated laser beams without the appropriate polarization compensation. Modulation contrast is also dependent on the spectral bandwidth and angular width (F#) of the incident light, with contrast generally dropping as the bandwidth is increased or the F# is decreased. Modulation contrast within LCDs can also be reduced by residual de-polarization or mis-orienting polarization effects, such as thermally induced stress birefringence. Such effects can be observed in the far field of the device, where the ideally observed “iron cross” polarization contrast pattern takes on a degenerate pattern.
As is obvious to those skilled in the digital projection art, the optical performance provided by a LCD based electronic projection system is, in large part, defined by the characteristics of the LCDs themselves and by the polarization optics that support LCD projection. The performance of polarization separation optics, such as polarization beamsplitters, pre-polarizers, and polarizer/analyzer components is of particular importance for obtaining high contrast ratios. The precise manner in which these polarization optical components are combined within a modulation optical system of a projection display can also have significant impact on the final resultant contrast.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates well only with incident light over a limited range of angles (a few degrees). Because the MacNeille prism design provides good extinction ratio for one polarization state only, a design using this device must effectively discard half of the incoming light when this light is from an unpolarized white light source, such as from a xenon or metal halide arc lamp.
Conventional glass polarization beamsplitter design, based on the MacNeille design, has other limitations beyond the limited angular response, including fabrication or thermally induced stress birefringence. These effects, which can degrade the polarization contrast performance, may be acceptable for mid range electronic projection applications, but are not tolerable for cinema projection applications. The thermal stress problem has been improved upon, with the use of a more suitable low photo-elasticity optical glass, disclosed in U.S. Pat. No. 5,969,861 (Ueda et al.), which was specially designed for use in polarization components. Unfortunately, high fabrication costs and uncertain availability limit the utility of this solution. As a result of these problems, the conventional MacNeille based glass beamsplitter design, which works for low to mid-range electronic projection systems, operating at 500–5,000 lumens with approximately 800:1 contrast, falls short for digital cinema projection.
Other polarization beamsplitter technologies have been proposed to meet the needs of a LCD based digital cinema projection system, such as the bi-directional total internal reflection beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.) and liquid-filled beamsplitters (see U.S. Pat. No. 5,844,722 (Stephens et al.)). However, in recent years, successful LCD based projectors have been built around wire grid polarizers. Wire grid polarizers have been in existence for many years, and were primarily used in radio-frequency and far infrared optical applications. Use of wire grid polarizers with visible spectrum light has been limited, largely due to constraints of device performance or manufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.) discloses use of a wire grid beamsplitter in a virtual image display apparatus, which has high light efficiency but very low contrast (6.3:1). A second wire grid polarizer for the visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada). While the device discussed by Tamada provides polarization separation, the contrast ratio is inadequate for cinematic projection and the design is inherently limited to rather narrow wavelength bands.
Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.); U.S. Pat. No. 6,243,199 (Hansen et al.); and U.S. Pat. No. 6,288,840 (Perkins et al.), high quality wire grid polarizers and beamsplitters have been developed for broadband use in the visible spectrum. These new devices are commercially available from Moxtek Inc. of Orem, Utah. While existing wire grid polarizers, including the devices described in U.S. Pat. Nos. 6,122,103 and 6,243,199 may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices do have a number of advantages. When compared against standard polarizers, wire grid polarization devices exhibit relatively high extinction ratios and high efficiency. Additionally, the contrast performance of these wire grid devices also has broader angular acceptance (NA or numerical aperture) and more robust thermal performance with less opportunity for thermally induced stress bireftingence than standard polarization devices. Furthermore, wire grid polarizers themselves are more robust than are conventional absorptive polarizers relative to harsh environmental conditions, such as light intensity, temperature, and vibration. While generally these commercially available wire grid devices perform well across the visible spectrum, an innate fall off in the polarization response for blue can mean that the blue channel may require additional contrast enhancement to match the red and green for demanding applications.
Wire grid polarization beamsplitter (PBS) devices have been employed within some digital projection apparatus. For example, U.S. Pat. No. 6,243,199 (Hansen et al.) discloses use of a broadband wire grid polarization beamsplitter for projection display applications. U.S. Pat. No. 6,234,634 (also to Hansen et al.) discloses a wire grid polarization beamsplitter that functions as both polarizer and analyzer in a digital image projection system. U.S. Pat. No. 6,234,634 states that very low effective F#'s can be achieved using wire grid PBS, although with some loss of contrast. Notably, U.S. Pat. No. 6,234,634 does not discuss how the angular response of the wire grid polarizers can be enhanced, nor how polarization compensation may be used in combination with wire grid devices and LCDs, to reduce light leakage and boost contrast, particularly for fast optical systems operating at low F#'s.
Of particular interest and relevance for the apparatus and methods of the present invention, it must be emphasized that individually neither the wire grid polarizer, nor the wire grid polarization beamsplitter, provide the target polarization extinction ratio performance (nominally >2,000:1) needed to achieve the desired projection system frame sequential contrast of 1,000:1 or better, particularly at small F#'s (<F/3.5). Instead, both of these components provide less than ˜1,200:1 contrast under the best conditions. Significantly, performance falls off further in the blue spectrum. Therefore, to achieve the desired 2,000:1 contrast target for the optical portion of the system (excluding the LCDs), it is necessary to utilize a variety of polarization devices, including possibly wire grid polarization devices, in combination within a modulation optical system of the projection display. However, the issues of designing an optimized configuration of polarization optics, including wire grid polarizers and polarization compensators, in combination with LCDs, color optics, and projection lens, have not been completely addressed either for electronic projection in general, or for digital cinema projection in particular. Moreover, the prior art does not describe how to design a modulation optical system for a projection display using both LCDs and wire grid devices, which further has polarization compensators to boost contrast.
There are numerous examples of polarization compensators developed to enhance the polarization performance of LCDs generally, and vertically aligned LCDs particularly. In an optimized system, the compensators are designed to enhance the performance of the LCDs and of the polarization optics in combination. These compensators typically provide angularly varying birefringence, structured in a spatially variant fashion, to affect polarization states in portions (within certain spatial and angular areas) of the transiting light beam, without affecting the polarization states in other portions of the light beam. As a first example, U.S. Pat. No. 4,701,028 (Clerc et al.) discloses birefringence compensation designed for a vertically aligned LCD with restricted thickness. As another example, U.S. Pat. No. 5,039,185 (Uchida et al.) discloses a vertically aligned LCD with compensator comprising at least two uniaxial or two biaxial retarders provided between a sheet polarizer/analyzer pair. Additionally, U.S. Pat. No. 5,298,199 (Hirose et al.) discloses the use of a biaxial film compensator correcting for optical birefringence errors in the LCD, used in a package with crossed sheet polarizers, where the LCD dark state has a non-zero voltage (a bias voltage).
Compensators can be complex structures, comprising one or more layers of films, optical adhesives, and other materials. For example, U.S. Pat. No. 5,619,352 (Koch et al.) discloses compensation devices, usable with twisted nematic LCDs, where the compensators have a multi-layer construction, using combinations of A-plates, C-plates, and O-plates, as needed.
Polarization compensators can also be designed to correct for both the vertically aligned LCD and the polarization optics in combination. Most of these prior art compensator patents discussed previously, assume the LCDs are used in combination with sheet polarizers, and correct only for the LCD polarization errors. However, polarization compensators have also been explicitly developed to correct for non-uniform polarization effects from the conventional Polaroid type dye sheet polarizer. The dye sheet polarizer, developed by E. H. Land in 1929 functions by dichroism, or the polarization-selective anistropic absorption of light. Compensators for dye sheet polarizers are described in Chen et al. (J. Chen, K.-H. Kim, J.-J. Kyu, J. H. Souk, J. R. Kelly, P. J. Bos, “Optimum Film Compensation Modes for TN and VA LCDs”, SID 98 Digest, pgs. 315–318.), and use a combination A-plate and C-plate construction. Similarly, U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses a compensator constructed for use in projector apparatus using an LCD with the conventional MacNeille prism type polarization beamsplitter. This compensator comprises a ¼ wave plate for compensating the prism and an additional 0.02 λ's compensation for the inherent LCD residual birefringence effects.
While this prior art material extensively details the design of polarization compensators used under various conditions, compensators explicitly developed and optimized for use with wire grid polarizers and vertically aligned LCDs are not disclosed in the prior art. In order to achieve high brightness levels, it is most advantageous for an optical system to have a high numerical aperture (>˜0.13), so that it is able to gather incident light at larger oblique angles. The conflicting goals of maintaining high brightness and high contrast ratio present a significant design problem for polarization components. Light leakage in the OFF state must be minimal in order to achieve high contrast levels. Yet, light leakage is most pronounced for incident light at the oblique angles required for achieving high brightness.
Commonly-assigned U.S. Pat. No. 6,585,378 (Kurtz et al.) discloses a projection system employing wire grid polarizers and polarization beamsplitters. This disclosure also acknowledges the potential need for polarization compensation in optical systems having wire grid polarization beamsplitters and LCDs, but does not teach the design thereof. However, as is disclosed in commonly-assigned U.S. Patent Application Publication No. 2003/0128320, polarization compensators have been developed and optimized for wire grid polarizers and polarization beamsplitters. In particular, this application describes compensators designed for the wire grid devices, as well as compensators for wire grid devices that also work with vertically aligned LCDs and with compensators for vertically aligned LCDs. It has been shown that a modulation optical system comprising wire grid polarizers, a wire grid polarization beamsplitter, a vertically aligned LCD, and a customized polarization compensator, can provide polarization contrast in excess of the 1,000:1 target across a wide range of incident angles (small F's).
This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/163,228 by Silverstein et al. which introduces the concept of in-plane rotation of a wire grid polarization beamsplitter by a fixed amount as a means for polarization compensation. As this rotation enables the wire grid polarization beamsplitter to effectively substitute for an A-plate compensator by providing an equivalent in-plane retardance, the polarization compensator needed by the projector can be simplified. In particular, the fabrication of standard polarization compensators used in such a system can be difficult, as, depending on the compensator design, specific values and orientations of retardance are required, and are assembled from a combination of existing materials. Typically these materials are thin film sheets, such as polycarbonate or acetate, whose optical retardance depends both on material properties and film fabrication methods. Compensators can then be assembled by stacking an appropriate combination of these films between glass plates, with intervening layers of optical adhesive to provide optical index matching. The assembled compensator must be free from both dirt and bubbles, and provide a consistent spatially uniform retardance while under a large heat (light) load. Alternately, a compensation layer with a nominal target retardance can be spun coated directly on a glass substrate, thereby potentially simplifying the construction of the compensator device. However, the construction of compensators that require multiple retardation layers with different properties can still be difficult. Furthermore, the optimum retardance required to correct for the inherent residual birefringence (such as the 0.02 λ's mentioned previously) can vary significantly from device to device. Ideally, but likely impractically, this implies that to maximize contrast from device to device would require matching each LCD with an appropriately optimized compensator. Polarization compensators can also be fabricated by alternate technologies, including crystalline sheets, optical thin films, liquid crystal polymers, and sub-wavelength dielectric form birefringent structures. Nonetheless, providing the equivalent of the A-plate portion of a compensator by means of wire grid polarization beamsplitter polarization rotation can be a significant improvement and cost savings.
In that case, there is then a need to provide a mechanical housing that holds the wire grid polarization beamsplitter in proximity to a liquid crystal display and other nearby polarization components, which further provides for the controlled in-plane rotation of the wire grid polarization beamsplitter, without disturbing other key aspects of its alignment or operation. Commonly-assigned copending U.S. patent application Ser. No. 10/624,346 by Ehrne et al. provides a housing and method for mounting polarization components and a reflective LCD spatial light modulator. In particular, this application provides a mechanical housing or frame that holds a wire grid polarization beamsplitter in position relative to an LCD in a high lumen, high heat load projector, while minimizing thermal distortion and stress birefringence. Among key design considerations discussed in this application are the mounting a wire grid polarizing beamsplitter while maintaining the surface of this component at an accurate 45 degree orientation relative to both the surface of the spatial light modulator and the surface of an analyzer. A related problem that must be resolved in electronic projection apparatus design is alignment of the spatial light modulator itself relative both to the wire grid polarizing beamsplitter and to the projection optical path. Maintaining precision alignment without the negative effects of thermal drift is a key design goal for high-end electronic projection apparatus. This application however does not provide means to mechanically rotate the wire grid polarization beamsplitter by a fixed amount to enable polarization compensation, allowing in-plane rotation without causing undesired constraints, stresses or movements that could effect the wire gird polarization beamsplitter, such as under a high heat load or during the transition from room ambient to high heat load operation.
Other documents in the art, such as an article in the SID 02 Digest entitled “The Mechanical-Optical Properties of Wire-Grid Type Polarizer in Projection Display System” by G. H. Ho et al., which presents some of the key design considerations for deploying wire grid polarizer components in imaging apparatus using reflective LCD spatial light modulators, do not disclose the need or the mechanical means for rotating a wire grid polarization beamsplitter in-plane. Likewise, U.S. Patent Application Publication No. 2003/0117708 (Kane), which discloses a sealed enclosure comprising of a wire grid polarizing beamsplitter, a spatial light modulator, and a projection lens having the interior space filled with a inert gas or vacuum, also does not consider the need or mechanism for providing controlled in-plane rotation of wire grid polarization beamsplitter as part of a housing design.
Therefore, as can be seen, there is a need for a mechanical housing of an LCD based electronic projector in which controlled in-plane rotation of a wire grid polarization beamsplitter is provided, so as to enable an alternate polarization compensation means.